1. Anthony R. Lupo Department of Soil, Environmental, and Atmospheric Science 302 E ABNR building University of Missouri – Columbia Columbia, MO 65211

2.  Atmospheric blocking is a phenomenon that may occur relatively rarely (compared to smaller scale phenomenon), but may dominate a region’s weather for a whole season.  Summer 2010 was a prime example as blocking dominated Russia’s weather from May to mid- August.

6.  There is not even a set definition, or criterion for blocking that is commonly accepted in meteorology today.  A generic definition may be: a long-lived, mid- latitude, mid-tropospheric positive geopotential anomaly.

7.  The basic climatological characteristics of blocking have been well known since the 1950’s (e.g., Rex, 1950). That is, blocking is generally a winter season feature that occurs downstream of the storm tracks over the oceans.  This presentation will focus on what new contributions were made by the Climate Change Group at the University of Missouri and the A.M. Obukhov Institute for Atmospheric Physics

8.  Friday 6 March, 2015 – Moscow, Russia  (left) Mokhov (IAP) (Right) Lupo (MU)

9.  Blocking is still not a well – forecast problem, especially at onset and termination. The processes are only somewhat understood.  Blocking can be responsible for economic (e.g. agricultural, capital, and infrastructure) and societal losses (e.g. cost of human life), directly and indirectly.

10.  The work of Lupo and Smith (1995a) demonstrated that all blocking events are accompanied by a strong developing cyclone.  They did not propose the idea of upstream, precursor cyclones (e.g., Kalnay, 1979; Frederiksen, 1982; Colucci,1985; Mullen 1986,1987; Agayan and Mokhov, 1989), but demonstrated that these were present in every case, and they aid in maintaining the block.

12.  They also demonstrated that there was a correlation between the vigour of mid-latitude cyclone development and block intensity and duration.  Agayan and Mokhov (1989) was the first to demonstrate that blocking was an important contribution to Northern Hemisphere continental climate regimes. Lupo and Smith (1995) used this idea to construct a separate climatological category.

13.  Lupo and Smith (1995) developed the first intensity index for blocking and based this on the strength of mid-latitude height gradients. This index was refined by Wiedenmann et al. (2002) [Lupo, Mokhov, Tikhonov].  Lupo, Oglesby, and Mokhov (1997) were the first to propose that blocking characteristics may change with climate (become more persistent, occur more often, but weaker overall)

15.  In a paper from MU – and IAP (Wiedenmann et al. 2002) our research was the first to demonstrate that the occurrence and strength of blocking could be correlated to interannual climate variations.  We showed that blocking was more frequent and intense during La Niña (El Niño) years in the Northern (Southern) Hemisphere. This result could be linked to cyclone variability.

16.  We also demonstrated that up until 1999, the long term trend in the SH was for fewer blocking events, and this could be related to the increasingly zonal SH flow in the mid- latitudes.  New analysis (Lupo et al. 2008) suggests blocking to be on the increase again globally and this may be related to the Pacific Decadal Oscillation in the SH, or climate change.

17.  Early views of blocking recognized it to be the result of large-scale waves becoming quasi- stationary due to resonance with topographical features.  Some studies, such as Charney and Devore (1979), for example, considered that blocking may be a distinct atmospheric flow regime (energy state) different from that of non- blocked flow.

18.  Regardless, blocking was thought to be the result of hemispheric-wide flow processes.  The contribution of synoptic-scale cyclones was speculated by early pioneers such as Frederiksen, Shutts, or Kalnay and Merkine. These studies used analytical or numerical models. 

19.  Nonetheless, these earlier works suggested that the storm track (small-scale cyclones) would support block formation so long as the cyclones were upstream of the block.  In the 1980s, work by Colucci and Mullen suggested that as long as the cyclone wave was about ¼ wavelength (90 o out of phase) upstream, the blocking event would form.

20.  Lupo and Bosart (1999) further state, that not only is the ¼ wavelength necessary, but that;  Synoptic scale forcing too far upstream will not support the blocking event.  Synoptic scale forcing closer than this can lead to weaking of the blocking event.

21.  This view point of blocking suggests that local forcing was the key to the development of blocking events, not hemispheric-wide forcing.  This hypothesis was supported by the statistical analysis of Legenas and Okland (1983) who suggested that if blocking were forced by hemisphere –wide, there should be a greater frequency of occurrence of simultaneously occurring events.

22.  Tsou and Smith (1991) used the height tendency equation to examine the growth of individual blocking events in a similar manner to that of mid-latitude cyclones.  Lupo and Smith (1995) then used a modified form of the vorticity equation to examine what processes contribute to block formation, maintenance and decay. 

23. As in the study of cyclones, these techniques treat blocking events as distinct events. The type of equations used combines the Vorticity Equation with the First Law of Thermodynamics.

25.  The key result of this study was to demonstrate that the amplification of the synoptic-scale wave strengthens the anticyclonic vorticity field and transport into the blocked region.  This occurred not only during block formation, but helped to maintain it.

27.  They also showed that the transport of cold air in the lower troposphere into the anticyclone region by a synoptic-scale transient contributed to the decay of the event.  This was the first study to suggest upstream transients could also force decay.

29.  This group then turned their attention to the Southern Hemisphere.  Why are there so few events there?  Why do they translate more than in the NH?

30.  Burkhardt and Lupo (2005) examine SH blocking, and in particular look at the planetary and synoptic scale interactions using the Potential Vorticity equation:  They find that blocking in the SH results primarily from the superposition of large and small scale waves rather than interaction as in NH.

31.  None of these studies has solved the problem of poor forecasts for the development and decay of blocking.  Thus, MU (Lupo) joins the IAP (Mokhov) again! - Izvestiya) utilized the “Dymnikov conjecture” to examine flow stability.

32.  This relationship should be valid for large-scale flows that are non-divergent and non- deformational.  The main results of this work demonstrated that blocking, once formed, can be relatively better predicted,  but that the large-scale flow can become unstable, destroying the event.

33.  They (Lupo and Mokhov) also show that blocking will decay under four scenarios  1. The active role of the synoptic transients as suggested by Bosart and Lupo (1999)  2. No positive or negative support from the synoptic scale (Bosart and Lupo, 1999)  3. and 4. Same as 1and 2, except with abrupt change in the large scale.

34.  Hussain and Lupo (2010) examined 126 NH blocking events over a three-year (2002-2004) period using the Dymnikov relationship. In every case, the development and decay were associated with abrupt changes in area averaged enstropy.  The area integrated enstrophy may have value as a tool for predicting the formation and decay of blocking.

35.  In mid-to-late January a very strong and persistent blocking event formed in the Gulf of Alaska. During part of the life cycle the hemispheric flow regime was highly amplified, and during the other part less amplified.

36.  The intense Pacific Blocking event was undergoing a second period of intensification after onset, which coincided with the flow regime transformation. This may have allowed the event to survive.

37.  Through flow transition then Decay……….

38.  The research efforts of both the MU (USA) and IAP (Russia) groups has led to advances in the understanding of the climatological and dynamic character of blocking.  In spite of these advances, several questions remain including the forecasting of development and decay, and how the nature of blocking may change as climate changes.

39. Class of 1988! (Rules?)

40.  Questions?  Comments?  Criticisms?  Email: lupoa@missouri.edu